1. Field of the Invention
The present invention relates generally to antennas and more particularly to phased array antennas.
2. Description of the Related Art
The antenna beam of a phased array antenna points in a direction that is normal to its phase front. In one conventional type of antenna, a plurality of phase shifters are incorporated in the array so that the phase front can be steered as desired. In another conventional type of antenna, beam-steering phase shifts are realized by varying the frequency of signals that are transmitted through time delay elements. This latter process scans the antenna beam but typically has the disadvantage that the frequency of the radiated beam is dependant upon the beam's pointing direction.
In contrast, U.S. Pat. No. 3,090,928 (issued May 21, 1963 in the name of William R. Welty) described the use of two time delay elements (e.g., tapped delay lines) to realize beam steering with a constant radiated frequency. In this patent, a reference signal is sent down one delay line and a tuning signal is sent down the other delay line. Corresponding tap points on the delay lines are coupled to mixers and the mixer outputs are coupled through filters to array radiators. The filters are configured to pass a desired mixing product (e.g., a difference signal) and block the reference and tuning signals.
U.S. patent application Ser. No. 08/711,428 (entitled "Simultaneous Multibeam and Frequency Active Photonic Array Radar Apparatus" and filed Sep. 5, 1996) is directed to active array systems that process multiple beams over a wide frequency range. It replaces conventional electronic radio-frequency (RF) delay lines (e.g., those of U.S. Pat. No. 3,090,928) with fiber optic delay lines. The delay lines provide a wide operating frequency range and the ability to process different RF signals through the use of light signals that have different wavelengths. Accordingly, an RF signal that is modulated on one light carrier does not interact with another RF signal that is modulated on a second light carrier. The RF signals may be placed on and taken out of the fiber delay lines using optical filtering (e.g., wavelength division multiplexing) of different light carriers.
A basic transmit manifold is described which has RF oscillators that generate a tuning frequency and a signal frequency. The transmit manifold also includes a solid state light source and electro-optic modulators that are coupled to an input of an optical manifold which is formed by a plurality of optical fibers. The output of this optical manifold is connected through photodiode detectors to array radiators of which each includes a series combination of a mixer, a filter and an RF amplifier.
A transmit implementation is shown that can provide two transmitted beams. With the exception of the optical manifold, this implementation duplicates all of the above-recited structures. In addition, pairs of wavelength division multiplexing (WDM) devices interface the optical manifold to the other structures. Accordingly, an active array system with multiple radiated beams can be realized but only at the expense of multiple duplications of photonic and electronic modules. It is stated that this duplication minimizes unwanted mixing products. However, this duplication also causes significant increases in size, weight and cost of the active array system.